How to make two internal SSD drives work best on a Mac

I have a mid-2010 17" MacPro with two internal 1TB SSD drives, running Catalina.

My hope was that I might be able to figure out a way to make them work effectively as one by pointing the "Documents", "Pictures" or "Movies" main folders to wherever I have space, much in same way as you can by setting up preferences in, for example, an app like iTunes. Not being able to do so means I have one SSD drive almost full and the other largely empty.

My question is: Has anyone else found a way to organize files in such a way that the Mac treats the two internal SSD drives effectively as one?

You can set Photos.app to have its library at a given location in its preferences.

As for Documents: there's nothing to stop you just storing all (or some) of your documents on a folder on another drive. You can even put a folder in the Finder sidebar to the location to make it easy to navigate to.

Files don't need to go in the Documents folder, and there's nothing special about it (apart from annoying security requests in Catalina and up).

I haven't tried this myself but you should be able to create an APFS volume group from your two drives to effectively create one "Fusion" drive. You would use

sudo diskutil apfs create

or

sudo diskutil apfs createContainer

to accomplish this. You can run those commands as-is to read usage. Also see the manual page for diskutil (man diskutil).

The advantage of a Fusion drive is that you don't have to manually manage which files get put onto which drive; the whole thing behaves like a RAID 0 array (i.e., JBOD, "Just a Bunch of Disks"). The downside is that you're now multiplying your chances of total data loss if there's corruption on either of the drives. So before beginning, you are very strongly advised to create AND maintain a backup of your data.